Building a high-transmission-loss enclosure around a distributed noise source is a method of reducing the sound radiated from the source. For these enclosures to be effective at low frequencies, active control is often preferable to strictly passive means due to the increased size, weight or cost of passive methods effective against frequencies.
Often noise sources are surrounded by an "enclosure" that is inherent to the noise source. One example is a power transformer. In this case, the transformer core is placed in a tank filled with oil. The transformer core causes the outside surfaces of the tank to vibrate and radiate acoustic energy. Thus the outside surfaces of the tank are the noise source. These tanks are large. For example, the tank for a small transformer (e.g., 7.5 MVA) may be about 8 ft. wide, 4 ft. deep and 10 ft. tall. Some large transformers are rated for a few hundred MVA and are many times larger than those dimensions. Active control of noise radiating from transformer tanks has been attempted by many people for decades. See for example Conover (U.S. Pat. No. 2,776,020), or more recently O. L. Angevine as described in "Active Cancellation of the Hum of Large Electric Transformers", Proceedings of Inter-Noise, Jul. 20-22, 1992. These prior attempts have been able to decrease noise levels for only a narrow angle radiating from the transformer. One reason for their inability to obtain global noise reduction stems from their inability to "couple tightly" to the noise source. By "couple tightly" it is meant that the anti-noise sources closely match the location, distribution and levels of the noise sources. One way to improve the source coupling is to use low profile acoustic sources; for example, thin panels driven by piezos. In this case thin, large-area piezo ceramic patches operating in the d.sub.31 mode (i.e., piezoceramics apply a moment-load to the panel) are attached directly to the surface. If these low-profile sources are attached closely to the structure and the noise source is not overly complex in its distribution, then large global reductions are achievable. However, designing and building many linear, high-level, low-frequency sources can be prohibitively expensive for complicated noise sources.
A preferable method of solving the coupling problem is to place piezo-ceramic patches directly on the surface of the panel or enclosure. One patch is placed at the peak of each half wavelength for those acoustically-radiating structural modes which have resonant frequencies near the excitation frequency(s). This ensures optimal coupling as far as location and distribution of the actuators. However, obtaining adequate actuator output level is more difficult. This requires optimizing the piezo-actuator design, and performing impedance matching between the actuator and panel or enclosure.
Performing impedance matching between the piezo-actuator and the panel or enclosure can be difficult. Early research has established the optimal piezo thickness for the case of static deformation. For example, using simple static models, S. J. Kim and J. D. Jones (1990) showed that the piezo thickness for maximum actuator output for steel panels is about one-half the thickness of the steel panel to which it is attached. This is described in "Optimal Design of Piezo-actuators for Active Noise and Vibration Control", AIAA 13th Aeroacoustics Conference, October 1990. Increasing the piezo thickness beyond the optimal thickness actually decreases the effective actuator output. Current research is focusing on the optimal piezo thickness for the dynamic case as, for example, C. Liang, S. Fanping and C. Rogers, "Dynamic Output Characteristics of piezoelectric Actuators", Proceedings of Smart Structures and Materials, February 1993. Since most of this research has focused on aerospace applications, the panels have been thin and lightweight with low damping. In this case, impedance matching is straightforward since the piezoceramics are typically thin (e.g., 0.010 inch thick or less). An audio-amplifier can be used with a step-down audio-transformer run "backwards" (i.e., with the secondary side wired to the amplifier). Using this approach, peak voltages of only about 200 volts are required. Power requirements can be reduced further by adding an inductor in series or parallel with the piezo or using other components such that the circuit is driven in resonance. This piezo thickness (0.010 inch thick or less) can be attached to aluminum panels up to about 0.025 inch thick maximum, or to steel panels up to about 0.020 inch thick maximum with good mechanical impedance matching (according to static impedance matching methods). However, there are many industrial applications with thick, rigid panels or enclosures where the above approach will not work. For example, transformer tanks are typically 0.375 inch thick. This is 18 times thicker than the above approach would "allow".
Thus the above approach for electromechanical impedance matching is of little use for piezo-driven panels for industrial applications. For example, as stated above, transformer tanks are typically fabricated with 0.375 inch thick steel plate. This would imply an optimal piezo thickness of about 0.190 inch thick. The piezos can be driven continuously with up to 10 volts peak for every 0.001 inch of thickness. This would imply a drive voltage of 1890 volts peak. Amplifiers for such devices cost about $2000 per channel for large quantities of amplifier channels. Active control of the transformer tank may require up to 100 actuators, particularly for large transformers with high-order harmonics in their noise signature. This implies an amplifier cost of several hundred thousand dollars. This is the price of a passive solution for transformer noise: building a concrete bunker around the transformer, which has been used in Canada and Europe. Thus, in spite of the advantages of close coupling (location and distribution), applying thick piezos directly to the tank is not a practical solution with existing technology due to limitations with actuator level and high amplifier cost.